The present invention relates to the membrane separation of higher boiling point components from mixtures with a wide range of boiling points. In particular, the present invention is a membrane and process to separate aromatics from gasoline, or similar wide-boiling mixtures, such as petroleum naphtha.
Pervaporation is a well-known membrane process. Pervaporation has been and is being considered for recovery of aromatics from refinery streams. A multicomponent liquid feed may be separated based on a selective solution-diffusion mechanism, with the permeate removed as a vapor. A vacuum is typically maintained on the permeate side of the membrane to facilitate permeation. Pervaporation is an endothermic process. Heat input is required to maintain the pervaporation process. Typically, the feed to the pervaporation process is preheated to temperature selected for efficient permeation of the select portion of the feed into the membrane, and at a pressure sufficient to maintain the feed in liquid phase. The desired operating temperature and flux are maintained by heating the membrane and/or by reheating the feed. Adiabatic operation in conventional membrane separation systems can result in a significant drop in temperature and loss of permeate flux. Interstage reheating of the retentate/feed is conventionally used to maintain temperature. Adiabatic operation is very desirable, and most desirable without a significant drop in temperature.
Gasoline is a complex mixture of aliphatic and aromatic hydrocarbons having a wide boiling range. Aromatics may be separated from gasoline by pervaporation to obtain higher-octane fuel. However, the wide boiling range, variable composition and volatility of market gasolines make separation with simple pervaporation membrane systems difficult and inefficient. The lower boiling, lower octane aliphatic components in gasoline permeate competitively with the higher boiling higher-octane aromatics, thereby limiting the permeation of the aromatic content. The competitive permeation of aliphatic compounds also limits membrane selectivity, thereby reducing aromatic selectivity. High yields of aromatic permeate require considerable energy, which reduces the overall efficiency of the membrane system. Prior art membrane systems have also employed pre-distillation steps to remove lower boiling aliphatics from gasoline, thereby concentrating aromatics in the higher boiling liquid. Complex systems using pre-fractionation, multi-stage membrane processing, and/or recycle with post-fractionation, to address these issues are generally not desirable for efficient membrane systems.
The present invention enables considerable simplifications to the pervaporation process, when separating wide boiling range feeds such as gasoline for example. These simplifications can lead to the reduced cost, weight and volume and system complexity required for increased efficiency to enable commercialization of this application.